I was actually comparing the 5X to the MXPE, but some of reasons why I chose the MXPE should still hold true for you. Some may be subjective.
- First and foremost for me was size. I actually was more interested in the Nexus because it was smaller than the MXPE, and coming from an iPhone 4 I wasn't sure how I'd like a large phone. After getting it however, I'm pleasantly surprised how comfortable it is to use, even one handed. Much more so than other devices I've played with like my friends Note 5. I however don't think I'd want to go much bigger than the MXPE, therefore the 6P would be a definite no-go for me.
- One of the subjective points would be design of the device. I'm not a fan of the thin slab with the protruding camera bit. My wife has a Nokia 930 with similar design, although not as pronounced as the nexus devices. The added customization with Motomaker was a plus for me, as I have grown tired of the standard slabs of plastic or metal offered in 2 colors.
- Another subjective point is the Motorola features. For me it was mainly the MotoDisplay. Coming from another OS device, I wasn't sure how notifications and lock screen actions operated, but what I saw of the MotoDisplay intrigued me. the other Motorola features, like twist for camera, chop for flashlight have been nice. Haven't used MotoVoice yet though.
- Final point for me, and again purely subjective, is the brand. While I am a fan of Google and what they are doing with the Nexus Sphere, I am less impressed with past LG (5X) mobile devices, and have no idea who Huawei is (6P). I have however had 3 past Motorola devices and have been pleased.